EP0112726A2

EP0112726A2 - UV-stabilisation of oxymethylene copolymers

Info

Publication number: EP0112726A2
Application number: EP83307920A
Authority: EP
Inventors: Walter R. Bryant
Original assignee: Celanese Corp
Current assignee: Celanese Corp
Priority date: 1982-12-28
Filing date: 1983-12-22
Publication date: 1984-07-04
Also published as: EP0112726B1; CA1226393A; DE3380522D1; EP0112726A3; JPH0359101B2; JPS59133245A; US4446263A

Abstract

An improved moulding composition is provided which exhibits desirable weathering resistance and UV stability comprising an oxymethylene copolymer having at least one chain containing from about 85 to 99.6 mole percent of oxymethylene units interspersed with about 0.4 to 15 mole percent of oxyethylene units, about 0.1 to 1 percent by weight of 1,6--hexamethylene bis(3,5-di-tert-butyl)-4-hydroxy hydrocinnamate, about 0.015 to 0.4 percent by weight of 2,4,6-triamino-sym-triazine, about 0.1 to 0.75 percent by weight of bis(1,2,2,6,6-pentamethyl-4-piperidinyl)((3,5-bis(1,1 -di- i methylethyl-4-hydroxyphenol)methy4) butyl propanedioate and about 0.25 to 1.0 percent by weight of 2-hydroxy-4-n--octoxy-benzophenone.

Description

This invention relates to oxymethylene polymers having improved resistance to weathering and ultra-violet degradation.
Polyoxymethylene polymers having recurring -CH₂0- units have been known for many years. They may be prepared by the polymerisation of trioxane which is a cyclic trimer of formaldehyde. Polyoxymethylene varies in molecular weight, depending on its method of preparation.
High molecular weight solid polyoxymethylenes have been prepared by polymerising trioxane in the presence of certain fluoride catalysts such as antimony fluoride and may also be prepared in high yields and at rapid reaction rates by the use of catalysts comprising boron fluoride coordination complexes with organic compounds, as described in U.S. Patent No. 2,989,506 of Hudgin et al. Boron fluoride gas is also a rapid and effective catalyst, as disclosed in U.S. Patent No. 2,989,507 also of Hudgin et al. A group of oxymethylene polymers containing repeating carbon-to-carbon single-bonds in the polymer chain prepared, for example, by copolymerising trioxane and any of various cyclic ethers such as ethylene oxide or dioxalane is disclosed in U.S. Patent No. 3,027,352 of Walling et al. Such copolymers may be described as having at least one chain containing from 85 to 99.6 mole percent of oxymethylene units interspersed with from 0.4 to 15 mole percent of -O-R-units wherein R is a divalent radical containing at least 2 carbon atoms directly linked to each other and positioned in the chain between the two valences, any substituents on the R radical being inert.
Other methods of preparing oxymethylene polymers are disclosed by Kern et al in Angewandte Chemie 73(6) 177-186 (March 21, 1961), including polymers containing repeating carbon-to-carbon single bonds in the polymer chain by copolymerising trioxane with cyclic ethers such as dioxane, lactones such as beta-propiolactone, anhydrides such as cyclic adipic anhydride and ethylenically unsaturated compounds such as styrene, vinyl acetate, vinyl methyl ketone or acrolein.
Oxymethylene polymers, when exposed to ultra-violet light for long periods of time, undergo degradation which impairs their toughness and flexibility. Further, the oxymethylene polymers become badly discoloured when exposed to heat and ultra-violet light. The resulting brittleness and discoloration are undesirable, particularly in electrical and automotive applications where toughness, flexibility and colour retention over prolonged periods of time are required.
It is known that the susceptibility of oxymethylene polymers to weathering without undue decrease in thermal stability can be reduced by the addition of a 2-hydroxy benzophenone to a polyoxymethylene polymer wherein the polymer units are derived from cyclic ethers having at least two adjacent carbon atoms (see U.S. Patent No. 3,219,621 of Prichard et al).
It is desirable, however, to provide a polyoxymethylene moulding composition which exhibits improved resistance to weathering as well as improved UV stability.
It has now surprisingly been found that it is possible to provide such a composition in accordance with the present invention.
In accordance with the present invention there is provided a moulding composition which exhibits such stability comprising, based on the total composition:

(a) an oxymethylene copolymer having a melting point of at least about 150°C. and having at least one chain containing from 85 to 99.6 mole percent of oxymethylene units interspersed with from 0.4 to 15'mole percent of oxyethylene units;
(b) 0.1 to 1.0 percent by weight of 1,6-hexamethylene bis-(3,5-di-tert-butyl)-4 hydroxy hydrocinnamate;
(c) 0.015 to 0.4 percent by weight of 2,4,6-triamino-sym-triazine;
(d) 0.1 to 0.75 percent by weight of bis(1,2,2,6,6-pentamethyl-4-piperidinyl)((3,5-bis(1,1-dimethylethyl-4-hydroxyphenol)methyl)) butyl propanedioate; and
(e) 0.25 to 1.0 percent by weight of 2-hydroxy-4-n-octoxy-benzophenone.

The moulding compositionof the invention presents a number of advantages.
It is a polyoxymethylene moulding composition which exhibits desirable resistance to weathering.
It is a polyoxymethylene moulding composition which exhibits desirable UV stability.
It is a polyoxymethylene moulding composition which exhibits resistance to thermal, colour and surface embrittlement and degradation normally encountered upon exposure for prolonged periods of time to ultraviolet light under ambient conditions.
It has been surprisingly and unexpectedly found that the weathering and ultraviolet light (UV) stability of the polyoxymethylene copolymers of the present invention can be desirably enhanced by the presence of the additives of the present invention. The polyoxymethylene component and the weathering and UV stabilising components will be described in greater detail below.

A. The Polyoxymethylene Component

A"preferred group of polymers intended to be utilised in accordance with this invention is oxymethylene-cyclic ether copolymers having a structure comprising recurring units having formula (-O-CH₂-(G)_n-) wherein n is an integer from 0 to 5 and wherein n is 0 in from 60 to 99.6 percent of the recurring units.
A.preferred class of copolymers is those having a structure comprising recurring units having the formula (-O-CH₂-(CH₂)_n-) wherein n is an integer from 0 to 2 and wherein n is 0 in from 60 to 99.6 percent of the recurring units. These copolymers may be prepared by copolymerising trioxane with a cyclic ether having the structure
wherein n is an integer from 0 to 2.
Among the specific cyclic ethers which may be used are ethylene oxide, 1,3-dioxolane, 1,3,5-trioxepane, 1,3-dioxane, trimethylene oxide, pentamethylene oxide, 1,2-propylene oxide, 1,2-butylene oxide, neopentyl formal, pentaerythritol diformal, paraldehyde, tetrahydrofuran and butadiene monoxide.
The preferred catalysts used in the preparation of the desired copolymers are the boron fluoride coordinate complexes with organic compounds in which oxygen or sulfur is the donor atom. The coordination complexes of boron fluoride may, for example, be a complex with a phenol, an ether, an ester or a dialkyl sulfide. Boron fluoride dibutyl etherate, the coordination complex of boron fluoride with dibutyl ether, is the preferred coordination complex. The boron fluoride complex with diethyl ether is also very effective. Other boron fluoride complexes which may be used are the complexes with methyl acetate, with ethyl acetate, with Phenyl acetate, with dimethyl ether, with methylphenyl ether and with dimethyl sulfide.
The coordination complex of boron fluoride should generally be present in the polymerisation zone in an amount such that its boron fluoride content is from 0.0001 to 1.0 weight percent based on the weight of the monomers in the polymerisation zone. Preferably, amounts from 0.003 to 0.03 weight percent should be used in a continuous process.
The preferred catalyst used in preparing the oxymethylene copolymer is the aforementioned boron trifluoride as discussed in U.S. Patent No. 3,027,352 issued to Walling et al. Further information concerning the polymerisation conditions, amount of catalyst employed and so on is given in that Patent.
The monomers in the reaction zone are preferably anhydrous or substantially anhydrous. Small amounts of moisture, such as may be present in commercial grade reactants or may be introduced by contact with atmospheric air, will not prevent polymerisation but should be essentially removed for optimum yields.
In a specific embodiment of this invention, the trioxane, cyclic ether and catalyst are dissolved in a common anhydrous solvent, such as cyclohexane, and are permitted to react in a sealed reaction zone. The temperature in the reaction zone may vary from 0°C, to 100°C. The period of reaction may vary from 5 minutes to 72 hours. Pressures from subatmospheric to 100 atmospheres or more may be used, although atmospheric pressure is preferred.
It has been found that the relatively minor amounts of the cyclic ether other than trioxane used in the copolymerisation reaction generally disappear completely from the reaction mixture. The required ratio of trioxane to cyclic ether in the reaction mixture may therefore be roughly predetermined for a desired mole ratio in the polymer by assuming that all of the cyclic ether is used up and by assuming a particular conversion level from previous experience under substantially comparable conditions.
The chemical constitution of the cyclic ether must also be considered. Thus, 1,3-dioxolane contains both an oxymethylene group and an oxyethylene group. Its incorporation into the copolymer molecule introduces oxyethylene groups into the polymer molecule.
In general, the cyclic ether is present in the reaction mixture in the amounts from 0.2 to 30 mole percent, based on the total moles of monomer. The optimum proportion will depend on the particular copolymer desired, the expected degree of conversion and the chemical constitution of the cyclic ether used.
The copolymers produced from the preferred cyclic ethers in accordance with this invention generally have a structure substantially comprised of oxymethylene and oxyethylene groups in a ratio from 1000:1 to 6:1, preferably 250:1 to 1.5:1.
Upon completion of the polymerisation reaction, it is desirable to neutralise the activity of the polymerisation catalyst since prolonged contact with the catalyst degrades the polymer. The polymerisation product may be treated with an aliphatic amine, such as tri-n-butyl-amine, in stoichiometric excess over the amount of free catalyst in the reaction product, and preferably in an organic wash liquid which is a solvent for unreacted trioxane. Alternatively, if desired, the reaction product may be washed with water which neutralises catalyst activity. A detailed description of the methods of neutralising catalyst activity may be found in U.S. Patent No. 2,989,509 of Hudgin et al.
A preferred oxymethylene copolymer is commercially available from the Celanese Plastics and Specialties Company under the designation Celcon acetal copolymer. (Celcon is a Trade Mark).
The oxymethylene copolymers that are employed in the present invention are thermoplastic materials having a melting point of at least 150⁰C., and normally are millable or processable at a temperature of about 200°C. They generally have a number average molecular weight of at least 10,000. The preferred oxymethylene polymers have an inherent viscosity of at least 1.0 (measured at 60°C. in a 0.1 weight percent solution in p-chlorophenol containing 2 weight percent of alpha-pinene).
The oxymethylene copolymer component preferably is an oxymethylene copolymer that has been preliminarily subjected to a stabilising technique. Of course, it is desirable that the stabilisation be to a substantial degree. Such stabilising technique may take the form of stabilisation by degradation of the molecular ends of the polymer chain to a point where a relatively stable carbon-to-carbon linkage exists at each end. For example, such degradation may be effected by hydrolysis as disclosed in our United States Patent No. 3,219,623.
If desired, the oxymethylene copolymer may be endcapped by techniques known to those skilled in the art. A preferred end-capping technique is accomplished by acetylation with acetic anhydride in the presence of sodium acetate catalyst.

B. The Weathering and UV Stabilisation Components

It has been surprisingly found that the unique combination of additives employed in the moulding composition of the present invention enables highly desirable weathering resistance and UV stabilisation properties to be obtained.
Specifically, the composition comprises 0.1 to 1.0 percent, and preferably 0.2 to 0.5 percent by weight, of 1,6-hexamethylene bis-(3,5-di-tert-butyl)-4-hydroxy hydrocinnamate; about 0.015 to 0.4 percent, and preferably 0.1 to 0.4 percent by weight, of 2,4,6-triamino-sym-triazine, 0.1 to 0.75 percent, and preferably 0.15 to 0.35 percent by weight, of bis(1,2,2,6,6-pentamethyl-4-piperidinyl) ((3,5- bis(1,1-dimethylethyl-4-hydroxyphenol)methyl))butyl propanedioate; and 0.₀25 to 1.0 percent, and preferably 0.15 to 0.35 percent by weight, of 2-hydroxy-4-n-octoxy-benzophenone.
The 1,6-hexamethylene bis-(3,5-di-tert-butyl)-4-hydroxy hydrocinnamate additive serves as an antioxidant and is commercially available from Ciba-Geigy Corporation under the trade name Irganox 259. The bis(1,2,2,6,6-pentamethyl-4-piperidinyl) ((3,5-bis(1,1-dimethylethyl-4-hydroxyphenol)methyl)) butyl propanedioate additive serves as a free radical scavenger and is also commercially available from Ciba-Geigy Corporation under the trade name of Tinuvin 144. The 2,4,6- triamino-sym-triazine additive (i.e., melamine) serves as an acid scavenger. Oxymethylene copolymers described herein which incorporate the above three additives have been commercially available as moulding compositions from Celanese Corporation since about March, 1981.
The 2-hydroxy-4-n-octoxy-benzophenone additive serves as a UV absorber and is available from American Cyanamid Company under the trade name Cyasorb UV 531. The use of such an additive in oxymethylene moulding compositions is described in U.S. Patent No. 3,219,621. As demonstrated by the following Example, the combination of the 2-hydroxy-4-n-octoxybenzophenone UV absorber together with the three additives identified above provides highly desirable weathering resistance and UV stabilisation of the oxymethylene moulding compositions of the present invention.
The additives may be mixed with the copolymer by any suitable technique, e.g., by dry-blending in a Henschel mixer followed by melt extrusion and pelletising; by milling between two heated rolls and chopping into moulding granules; or by milling in Banbury mixer or Brabender Plastograph.
Blends of the present invention may include 1 to 60 and preferably 50 to 60 percent by weight, based upon the total weight of the article, of a reinforcing agent. Representative fibres which may serve as reinforcing agents include but are not limited to glass fibres, graphitic carbon fibres, amorphous carbon fibres, synthetic polymeric fibres, aluminum fibres, titanium fibres, steel fibres, tungsten fibres, ceramic fibres, etc.
Blends of the present invention may include filler materials in amounts ranging from 1 to 50 percent by weight. Examples of filler materials include calcium silicate, silica, clays, talc, mica, polytetrafluoroethylene, graphite, alumina trihydrate, sodium aluminum carbonate, barium ferrite, pigments, etc.
The invention is illustrated by the following Example. It should be understood, however, that the invention is not limited to the specific details of the Example.

EXAMPLE

Oxymethylene moulding compositions were prepared comprised of a polyacetal moulding resin marketed by Celanese Plastics and Specialties Co. under the trade name CelqonR.The compositions included various additives to determine the effect of such additives upon the weathering and UV stability of the moulding compositions, with the following additives being employed in various combinations:

(a) Antioxidant 1,6-hexamethylene bis-(3,5-di-tert-butyl)-4 hydroxyhydrocinnamate (Ciba-Geigy Irganox 259)
(b) Acid Scavenger 2,4,6-triamino-sym-triazine (American Cyanamid melamine)
(c) UV Stabilisers
- n-hexadecyl-1,3,5-di-t-butyl-4-hydroxybenzoate (American Cyanamid Cyasorb.UV 2908)
- 2(2'-hydroxy-3',5'-di-t-amylphenyl)benzotriazole (Ciba-Geigy Tinuvin 328)
- 2-hydroxy-4-octoxy-benzophenone (American Cyanamid Cyasorb UV 531)
- 2-(2-hydroxy-5-t-octylphenyl(benzotriazole (American Cyanamid Cyasorb UV 5411)
- bis(2,2,6,6-tetramethyl-piperidinyl-4)sebacate (Ciba-Geigy Tinuvin 770)
- 2(2'-hydroxy-5'-methylphenylibenzotriazole (Ciba-Geigy Tinuvin P)
- . bis(1,2,2,6,6-pentamethyl-4-piperidinyl)((3,5-di-tert butyl)-4-hydroxybenzyl)butyl propandioate (Ciba-Geigy Tinuvin 144)
(Irganox, Cyasorb and Tinuvin are Trade Marks)

Polyacetal moulding compositions were prepared containing certain of the above additives as described in the Tables and extruded at 194°C. and moulded at 194°C. to form 2 x 3 inch (5.1 x 7.6 cm) colour chips and tensile bars. The chips were placed in an Atlas Fadeometer at 92°C. and in a Q.U.V. Accelerated Weathering Tester with four hour condensation cycles for two weeks of ultra-violet light exposure. The condensation cycles were such as to allow the water to penetrate the test colour chips thereby promoting internal oxidation, blistering, microcracks and surface dulling. The results of the tests are summarised in Table I which demonstrates the results of using the additives in non-pigmented polyacetal resins as well as Table II which demonstrates the results of using the additives in pigmented polyacetal resin (336 hours of exposure). Colour differences and yellowness index are based on the Hunterlab colour rating system, with colour differences indicating total colour change while the yellowness index indicates the degree of yellowness subsequent to exposure. Yellowness index is determined according to ASTM (D1925)35. The light and water exposure apparatus is of the fluorescent UV-condensation type and employed according to ASTM G53-77 (QUV) as well as the carbon arc type employed according to ASTM 1499-64 (Atlas Fadeometer).
The data in Table III demonstrate that the combination of the Cyasorb UV 531 and Tinuvin 144 additives provides highly desirable colour stability and polyacetal moulding compositions in comparison to the use of other types of stabilisers.
Further tests were undertaken to determine the ability of moulding compositions prepared according to the present invention to retain initial elongation at break properties subsequent to 1000 hours of weathering exposure (using both QUV and Atlas Fadeometer tests). The moulding composition is pigmented by the addition of carbon black in the amounts noted. The results of the tests are summarised in Table,IV,

Claims

1. A moulding composition based on

(a) an oxymethylene copolymer having a melting point of at least 150°C. and having at least one chain containing from 85 to 99.6 mole percent of oxymethylene units interspersed with from 0.4 to 15 mole percent of oxyethylene units;
wherein the moulding composition comprises, by weight thereof:

(b) 0.1 to 1.0 percent of 1,6-hexamethylene bis-(3,5-di-tert-butyl)-4 hydroxy hydrocinnamate;

(c) 0.015 to 0.4 percent of 2,4,6-triamino-sym-triazine

(d) 0.1 to 0.75 percent of bis(1,2,2,6,6-pentamethyl-4-piperidinyl)((3,5-bis(1,1-dimethylethyl-4-hydroxyphenol)methyl)) butyl propanedioate; and I

(e) 0.25 to 1.0 percent of 2-hydroxy-4-n-octoxy-benzophenone.

2. The composition of claim 1 wherein component (b) is present in an amount of 0.2 to 0.5 percent by weight.

3. - The composition of claim 1 or 2 wherein component (c) is present in an amount of 0.1 to 0.4 percent by weight.

4. The composition of any of claims 1 - 3 wherein component (d) is present in an amount of 0.15 to 0.35 percent by weight.

5. The composition of any of claims 1 - 4 wherein component (e) is present in an amount of 0.15 to 0.35 percent by weight.

6. The composition of any of claims 1 - 5 further comprising a filler.

7. The composition of any of claims 1 - 5 further comprising a reinforcing agent.